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Vitamin B-2 (riboflavin) is a water soluble, stable vitamin. It plays an important role in several enzymes and coenzymes as a hydrogen atom acceptor in various metabolic roles, including protein metabolism. It is a constituent of coenzymes involved in the release of energy from glucose and fatty acids in the mitochondria. It is important in DNA synthesis, and in the formation of red blood cells. Riboflavin is also takes part in the activation of vitamin B-6 and the conversion of folic acid to its coenzymes.

Method of Action

Riboflavin is absorbed through the walls of the small intestine. Food in the gastrointestinal tract increases the absorption of riboflavin; when ingested without food, only 15% of riboflavin is absorbed. Within the intestinal cells, much of the absorbed riboflavin is phosphorylated to flavin mononucleotide (FMN) and, subsequently, is carried by the blood to the tissues.

Free riboflavin in the blood is transported by blood albumin to the liver, where it is converted to flavin-adenine dinucleotide (FAD). Both FAD and FMN play important roles in catalyzing oxidation-reduction reactions in the cells, and are essential coenzymes in the oxidative phosphorylation which occurs in cellular mitochondria.

Riboflavin is a component of the enzymes L- and D-amino acid oxidase, which catalyze the oxidation of amino acids, thereby involving the vitamin in protein metabolism. Riboflavin plays a primary role in the conversion of folacin to its coenzymes, some of which are needed for DNA synthesis and cell proliferation. It also activates vitamin B-6 and may participate in red blood cell formation.

Loss of protein from the body is accompanied by a loss in riboflavin. Riboflavin is not stored in the body and must be supplied in the diet, as any excess is excreted in the urine.

Properties & Uses

Riboflavin has been used effectively in treating the following symptoms of deficiencies: soreness and burning of the lips, mouth, and tongue; fissures and cracks in the lips; a purple, swollen tongue; and possible anemia.

Riboflavin needs increase during growth, pregnancy, wound healing, and lactation. This phenomenon is related to the fact that riboflavin plays a role in protein metabolism, which is increased during these periods.

Consequence of Deficiency

Early signs of riboflavin deficiency are: soreness and burning of the lips, mouth, and tongue; fissures and cracks in the lips; a purple, swollen tongue; burning and itching of the eyes; photophobia; a loss of visual acuity; and possible anemia. Individuals who ingest a diet devoid of animal protein sources and green leafy vegetables are prone to a riboflavinosis, a severe riboflavin deficiency.

If the intake of riboflavin is low for several months, the following symptoms will appear: cheilosis; angular stomatitis (cracks in the skin at the corners of the mouth); a greasy eruption on the skin of the nasolabial folds and scrotum; capillary overgrowth around the cornea of the eye; and a purplish, swollen tongue.

Riboflavin deficiency rarely appears alone; most often it appears in conjunction with multiple nutritional deficiencies. This makes diagnosis quite difficult, especially as niacin, iron, and pyridoxine deficiencies mimic the symptoms of riboflavin deficiency. Ocular manifestations (photophobia, itching, burning, and redness of the eyes) are believed to be the earliest signs of riboflavin deficiency. Urinary excretion of riboflavin can confirm diagnosis.

Toxicity Factors

Riboflavin is not known to be toxic.

Recommended Dietary Allowance

ageRDA (mg)RNI (mg)
0-6 months 0.4 0.4
6-12 months 0.5 0.4
1-3 years 0.8 0.6
4-6 years 1.1 0.8
7-10 years 1.2 1.0
11-14 years 1.5 1.2
15-18 years 1.8 1.3
19-50 years 1.7 1.3
51+ years 1.4 1.3
11-50 years 1.3 1.1
51+ years 1.2 1.1
pregnancy 1.6 1.4
lactation(1st 6 months) 1.8 1.6
(2nd 6 months) 1.7 1.6

The body's riboflavin requirement is related to total energy needs, body size, metabolic rate, and rate of growth, all of which are related to protein intake. The lower the protein intake, the more riboflavin is excreted and lost. For practical purposes, the general RDA standard for riboflavin is stated as 0.6 milligrams per 1,000 kilocalories for all ages. Persons in certain risk groups or clinical situations may require increased riboflavin. This includes persons living in poverty or with bizarre food habits, with gastrointestinal tract diseases or chronic illnesses where appetite is poor and malabsorption exists, during wound healing, and during growth periods such as in childhood, pregnancy, and lactation.

For over thirty years, Recommended Daily Amounts has existed in the United Kingdom. It has been used to measure the adequacy of an individual's diet. However, in 1991 the Committee on Medical Aspects of Food Policy (COMA) gave forth a whole new set of figures upon the request of the Department of Health's Chief Medical Officer. Reference Nutrient Intake (RNI) is one of these sets collectively known as "Dietary Reference Values." RNI is an amount of a nutrient that is enough for almost every individuals, even someone who has high needs for the nutrient. This level of intake is, therefore, considerably higher than what most people would need. If individuals are consuming the RNI of a nutrient they are most unlikely to be deficient in that nutrient.

Food Sources

Brewer's yeastCereals (enriched)
Green leafy vegetablesMilk
Organ meatsYogurt



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